Webwork - 11:55 am on Jul 6, 2011 (gmt 0)
Hello rp. Welcome to WebmasterWorld.
Start by looking at past reported sales of similar domains at DNJournal and other sites that archive past reported domain sales.
If the domain is making regular income via type-in traffic you can build an argument for a baseline value equal to a multiple of the annual type-in traffic revenue. x3? x5? x10 revenue? That depends on the likelihood that whatever "generic thing/good/service" people are searching for will remain popular and commercially viable.
As always, one of the best guage of a domain's aftermarket value is the number and quality of past "meaningful inquiries" = concrete offers to buy.
When it comes to selling one can always start high and see where/how the market responds. You can always ask for $100K or $1million . . or more . . and see if the market ignores you, laughs, or counters with $500K. Of course, the person/party countering with $500K may just be "pulling your leg" (joking) . . because they think you're (whatever).